In recent years, as electrophotographic photoreceptor materials there have been widely known inorganic photoconductive substances such as .alpha.-selenium, cadmium sulfide and .alpha.-silicon and organic photoconductive substances such as poly-N-vinylcarbazole and polyvinyl anthracene. However, these photoconductive substances have not a few disadvantages in price, properties and toxicity.
A mode has been often proposed which comprises allowing two functions of such photoconductive substances, i.e., generation of charges and transportation of charges thus generated to be performed by separate organic compounds in order to eliminate these disadvantages and provide a higher sensitivity. In this mode, it is expected that the lamination of a substance which exhibits a high charge carrier generation efficiency (charge generating substance) and a substance which exhibits a high capability of transporting charges (charge transporting substance) can provide a high-sensitivity electrophotographic photoreceptor. However, it is not always possible to simultaneously attain those various properties required for the electrophotographic photoreceptor, that is, high surface potential, high charge retention ability, high light sensitivity and little residual potential.
Thus, as charge transporting substances there have been recently proposed heterocyclic, arylamine, hydrazone and thioether compounds. As thioether compounds there have been proposed styrylthioether and 4-arylbutadienylthioether derivatives as disclosed in JP-A-1-140162 (the term "JP-A" as used herein means an "unexamined published Japanese patent application") and styrylphenylthioether derivatives as disclosed in JP-A-62-134652.
However, any of electrophotographic photoreceptors comprising these thioether compounds as a charge transporting substance leaves to be desired in light sensitivity. The compounds as disclosed in JP-A-1-140162 are disadvantageous in that they can be difficultly dissolved in a binder polymer during the formation of a light-sensitive layer.
Thus, various electrophotographic photoreceptors comprising a function-separating type light-sensitive layer comprising a combination of a charge generating substance and a charge transporting substance have been proposed. However, any of these electrophotographic photoreceptors leaves to be desired in light sensitivity. Furthermore, these electrophotographic photoreceptors have practical disadvantages that when used repeatedly in accordance with an electrophotographic process, they are subject to reduction in the capability of recovering the original chargeability (light fatigue), rise in the residual potential or reduction in the life thereof. As electrophotographic mode copying machines and printers have recently showed a progress in printing speed and miniaturization, electrophotographic photoreceptors have been desired to exhibit a high-speed responce. However, no electrophotographic photoreceptors have satisfied these requirements.